Fix macOS build
Fix macOS build

- Ensure python is available https://github.com/travis-ci/travis-ci/issues/2312
- Only pip install --user on linux, macOS is in virtualenv
- Set PATH in a os-agnostic manner https://github.com/huonw/travis-cargo/pull/71

aidancully/rust,aidancully/rust,graydon/rust,rust-lang/rustfmt,graydon/rust,aidancully/rust,graydon/rust,SingingTree/rustfmt,rust-lang/rustfmt,aidancully/rust,SingingTree/rustfmt,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust